I'd go back to the service manual and see what it suggests. My thought is to replace cables from main board up to the display and boards inbetween display and mainboard. If that doesn't clear it up, replace the mainboard.
I have a Samsung LE55A966D1WXXE from 2009 that have got horizontal lines on the screen.
They are black and is on every 5:th row. Mostly on the right side and fading out on the left side.
I have changed the caps on the power board and have disconnected and connected the flex cables from the board to the screen.
Do any of you have ant suggestion where I should check?
Thanks for your help